The present invention relates to floating offshore production platforms and particularly to the provision for export production risers on floating offshore production platforms. The term "floating offshore production platforms" is used to refer to a production platform that utilizes self-buoyancy of the platform to support the above-water structure and equipment, including piping, machinery, crew quarters, etc., and therefore is subject to the wind, wave and current action of the open ocean. These platforms include tension leg platforms restrained in position by tendons that are coupled to foundation structures secured to the ocean floor and floating platforms that are anchored in position by conventional anchors and mooring lines. Also included in the category of floating offshore production platforms are compliant towers that utilize a long slender structural tower in which the fundamental natural period of vibration is much longer than the natural wave periods. These platforms, while anchored to the ocean floor by conventional piles driven into the earth, may be used in deeper waters than normal bottom-rounded platforms. The water depths for any of these platforms are generally beyond the depth at which divers can operate and all subsea operations must be remotely controlled and operated.
In bottom-supported platforms it has been the customary practice to install the production export risers by clamping them directly onto the sides of the platform and to use a gentle bend in the export riser at the ocean floor to connect it to the horizontal pipeline that is used to transport the production ashore. This type of export riser cannot be used with a movable platform since the constant moving of the platform would subject the connection between the vertical portion of the riser and the horizontal portion to excessive stresses. Further, except in the case of a compliant tower platform, there is not sufficient structure available to clamp the vertical portion of the riser to the platform as is done in the case of bottom-supported platforms.
One type of riser that is sometimes used in conjunction with a floating production platform is the vertical tensioned riser. This type of riser is suspended vertically from the platform and connected vertically to a subsea structure anchored to the ocean floor. Piping and connections which are part of this subsea structure connect the vertical riser to the horizontal pipeline conveying the production ashore. This system requires the installation of a subsea structure onto the seafloor and the make-up of mechanical connections between the pipeline and the structure and between the riser and the structure using remotely operated vehicles. Due to the remoteness of these connections, there is high probability of having trouble both with the make-up of the connections and subsequently with leakage. This type of structure is very difficult to service since even the servicing would require remotely operated vehicles.
Another possible solution to the problem would be the use of a system similar to single-point mooring buoys that are used for offloading tankers offshore, employing flexible hose between the single-point mooring buoy and the platform. Among the difficulties of this system are interference between the mooring lines for the buoy and the platform, if any, and the restriction of vessel movements around the vicinity of the platform since they have to remain clear of the single-point mooring buoy. Likewise, the flexible hoses used in this type of system are expensive, heavy, and susceptible to collapse failure, and are high maintenance items requiring constant surveillance and frequent replacement.